Almost all author-publisher contracts are exclusive. You can’t sell a book to Random House and then, after Random House has done the heavy lifting (market research, editing, cover design, marketing, etc.), turn around and sell the book to Simon & Schuster. That means as much as I’d like to, I can’t make the Mercury books available on the Nook.
As I’ve pointed out before, however, the Nook uses the Android O/S, which means that it there is no technological barrier to reading Mercury Falls on your Nook. B&N has deliberately crippled the device to prevent it from being able to run certain apps (including the Kindle app). This may seem like a Machiavellian move on B&N’s part, but remember that B&N isn’t selling a device so much as they are selling a way of buying things. Amazon is doing the same thing: they actually take a loss on every Kindle device they sell because they know that once you have the device, you’ll be hooked on the Amazon model. My personal opinion is that B&N doesn’t have a chance to win this war, which means that the purchase of a Nook is probably analogous to buying a Betamax VCR in 1980.
There is a third option, however: Buy a third party device that can display books in any format. For example, I often read on my Android phone, using the free Kindle app. The downside of this option is that you’re going to pay more for a comparable device, because a third party company can’t afford to take a loss on the sale.
The upshot is this: when you buy a Nook, you are buying into B&N’s content distribution model, and that model has no place for books that are published by Amazon. That’s something people should keep in mind when deciding what e-reader to buy.